1. Field of the Invention
The present invention relates to an image display apparatus for observing an enlarged virtual image of an image on image display means, as being set near an eye portion of an observer, and more particularly to an image display apparatus with a wide field angle and with an excellent image quality.
2. Related Background Art
As conventional image display apparatus to be set near the eye of observer, there are helmet mounted displays incorporated with a helmet, and head mounted displays smaller and lighter than those, a support member of which is mounted on the head. Either of these is so arranged that an image displayed on an image display device such as CRT, LCD, etc. is enlarged and displayed as a virtual image ahead of the observer through an observation optical system.
The conventional display apparatus of this type are roughly classified into whether an eyepiece optical system in the observation optical system uses a reflection optical system or not.
An example of the eyepiece optical system not using the reflection optical system is an optical system of electric viewfinder used in video cameras etc. The optical system of this type is so arranged that an image displayed on LCD etc. is enlarged and displayed as a virtual image for the observer, using an eyepiece lens.
Those using the reflection optical system are classified into whether the observation optical system is a coaxial system or a decentered system.
Well-known systems with the observation optical system being a coaxial system are those as disclosed in Japanese Laid-open Patent Application No. 3-39924 and U.S. Pat. No. 5,151,722. FIG. 1 shows a schematic drawing of an optical system of this type. Numeral 121 designates an image display device, 122 a relay optical system for relaying an image on the image display device 121 to form an intermediate image thereof, 123 an intermediate image surface located on an image-forming plane of the relay optical system, 124 a plane beam splitter, 125 a spherical, reflective surface, and 126 an eye point where the pupil of observer is located. The image displayed on the image display device 121 curved in a concave shape is relayed through the relay optical system 122 to form an aerial image on the intermediate image surface 123. This aerial image is guided to the observer's pupil located at the eye point 126, using the spherical, reflective surface 125 as an eyepiece optical system, whereby the observer can observe a virtual image farther than the spherical, reflective surface 125. On this occasion, the plane beam splitter 124 is so arranged that it reflects light beams from the aerial image on the intermediate image surface 123 toward the spherical, reflective surface 125 and that it transmits light beams reflected by the spherical, reflective surface 125.
As one with the observation optical system being a decentered system, U.S. Pat. No. 4,854,688 discloses an optical system in which the above, spherical, reflective surface is decentered.
Also, U.S. Pat. No. 3,787,109 discloses a system not using the plane beam splitter, which is an optical system arranged in such a manner that beams from a display surface in the image display means are guided to the observer's pupil by a reflective surface decentered from the optical axis of the pupil, without forming an intermediate image. This shows the image display apparatus in which beams from the display surface in the image display means are guided to the observer's pupil while being reflected and converged by a curved mirror of a concave surface and in which the shape of the concave, curved mirror is a part of a paraboloid of revolution and the center of display image is located at the focus of the paraboloid of revolution. Further, U.S. Pat. No. 3,833,300 discloses an optical system having an eyepiece system utilizing a reflective surface shaped in a paraboloid of revolution, as in U.S. Pat. No. 3,787,109, in which two reflective surfaces in the shape of paraboloid of revolution are provided for the left and right eyes, respectively, and are integral with each other.
The paper, D. J. Rotier, "Optical Approaches to the Helmet Mounted Display" (Proceedings SPIE Vol. 1116, Helmet-Mounted Displays, 1989, P14-18), discloses an optical system which forms an intermediate image of the display image, in which a part of the relay optical system, and the eyepiece optical system are constructed of a reflective surface using a partial surface shape of a same paraboloid of revolution. FIG. 2 shows the schematic structure of the optical system. In FIG. 2, numeral 131 designates image display means, 132 a refracting optical system, 133 a reflective surface in the shape of a paraboloid of revolution, 134 a plane, reflective surface arranged to be perpendicular to an axis of revolution A of the paraboloid of revolution which is the reflective surface shape of reflective surface 133 and to include the focus B thereof, 135 a reflective surface arranged on the paraboloid of revolution being the reflective surface shape of the reflective surface 133 and in symmetry with the reflective surface 133 with respect to the axis of revolution A, and 136 an eye point where the observer's pupil is located. Beams from the image display means 131 are guided by the relay optical system of the refracting optical system 132 and the reflective surface 133 to form an image near the plane, reflective surface 134. Then beams from this image are guided to the eye point 136 by the eyepiece optical system of the reflective surface 135. In such a system, the optical system as described in "Optical Approaches to the Helmet Mounted Display" is so arranged that axial beams from the image display means 131 are focused at the focal point B on the plane, reflective surface 134 and then the observer observes a virtual image of the display screen at the infinity from the eye point 136.
Also, Japanese Laid-open Patent Application No. 2-297516 discloses a thinner structure in which the one plane, reflective surface in the above optical system as described in "Optical Approaches to the Helmet Mounted Display" is replaced by a plurality of plane, reflective surfaces.
Further, Japanese Laid-open Patent Application No. 5-134208 discloses a system in which beams from the display surface in image display means are relayed by a refracting relay optical system to form an intermediate image and this intermediate image is made to be observed through an eyepiece optical system having a reflective surface shaped in an ellipsoid of revolution. FIG. 3 shows the schematic structure of the system. In FIG. 3, numeral 141 designates image display means, 142 a refracting optical system, 143 an aperture of the refracting optical system, 144 a reflective surface in the shape of an ellipsoid of revolution, and 145 an eye point where the observer's pupil is located. Beams from the image display means 141 are guided by the relay optical system of the refracting optical system 142 to form an intermediate image. Beams from the intermediate image are guided by the eyepiece optical system of the reflective surface 144 to the eye point position 145. In such structure, the aperture 143 of the relay optical system is located at one focal point of the ellipsoid of revolution 144 being the reflective surface and the eye point 145 where the observer's pupil is located is located at the other focal point, thereby establishing a relation of pupil image formation between the pupil 143 of the relay optical system and the pupil at the eye point 145.
In the case of the structure of the optical system of electric viewfinder, information of external view cannot be provided to the observer, because the image display means is located on the optical axis of the observer's pupil. Also, because the focal length of eyepiece lens becomes shorter with an increase of field angle, the distance becomes shorter between the image display surface and the principal plane on the display surface side, of the eyepiece lens. In such structure, particularly where the image display means is one having contrast characteristics with directivity in the direction approximately perpendicular to the display surface, such as LCD, it is effective to use a concave lens having a diverging effect between the eyepiece lens and the display surface in order to provide images with wide field angle and with high contrast. This arrangement, however, had a problem that a distortion appeared with an increase of the field angle.
In the systems as disclosed in Japanese Laid-open Patent Application No. 3-39924 and U.S. Pat. No. 5,151,722, the plane beam splitter 124 needs to be set as inclined relative to the optical axis of the pupil between the spherical, reflective surface 125 and the observer's pupil. Since the distance is long between the eye point 126 and the spherical, reflective surface 125 being the eyepiece optical system, such an arrangement has a problem that the outer diameter of the spherical, reflective surface 125 becomes larger for wider field angles. In particular, where two image display apparatus are symmetrically arranged for binocular vision, physical interference would occur and thus this arrangement is not suitable for image display apparatus for wide field angle. Since beams emitted from the image display device 121 are subjected to reflection and transmission at the plane beam splitter 124, they lose a lot of light quantity there, thus causing a problem of failing to obtain a bright virtual image.
In U.S. Pat. No. 4,854,688, the above spherical, reflective surface is decentered relative to the optical axis of the pupil. If two image display apparatus are symmetrically arranged for binocular vision in this arrangement, end faces in the direction of the symmetry axis, of the spherical, reflective surfaces become closer to the observer's pupils. Thus, this arrangement can be thinner than the arrangement of Japanese Laid-open Patent Application No. 3-39924. However, because the plane beam splitter needs to be set between the spherical, reflective surface and the observer's pupil, the spherical, reflective surface cannot be set nearer to the pupil than the plane half mirror. Since the outer diameter of the spherical, reflective surface is determined by the distance from the eye point at a certain field angle, such structure does not allow the outer diameter of the spherical, reflective surface to be decreased. Therefore, this arrangement is not suitable for apparatus required to have a wide field angle. Also, the decentering of the spherical, reflective surface causes new decentering aberrations, but the arrangement of U.S. Pat. No. 4,854,688 failed to correct a decentering distortion and a decentering astigmatism. Another problem is a great loss of light quantity because it uses the beam splitter, as in Japanese Laid-open Patent Application No. 3-39924.
In U.S. Pat. No. 3,787,109 there is no optical member between the observer's pupil and the paraboloid-of-revolution mirror for guiding beams to the pupil, which can permit a so-called high eye point structure having a spatial room in the direction of the optical axis of the observer's pupil at the eye point. Since the paraboloid-of-revolution mirror reflects beams emerging from a focal point in the form of collimated beams, a virtual image can be displayed at infinity without spherical aberration. Thus, an image at the center of screen can be observed in a good state. However, off-axial beams emerging from positions away from the focal point of paraboloid of revolution are not converted into parallel beams by the paraboloid-of-revolution mirror and a decentering coma newly appears. Further, because there is no optical element for correcting aberrations generated by the paraboloid-of-revolution mirror, the off-axial beams will produce decentering aberrations such as a decentering curvature of field, a decentering distortion, etc. in addition to the decentering coma. Thus, only the image displayed near the focal point of paraboloid of revolution can be observed in a good state. Accordingly, there is a problem that a good virtual image cannot be displayed except for a narrow field angle. U.S. Pat. No. 3,833,300 also has the same problem.
With the optical system for helmet mounted display as described in "Optical Approaches to the Helmet Mounted Display," ignoring the spherical aberration in the refracting optical system 132, there occurs no spherical aberration in either of the reflective surfaces 133, 135 from the relation between the paraboloid of revolution and the focal point. Thus, correcting the spherical aberration in the refracting optical system 132, the spherical aberration by the overall system can also be corrected. Since off-axial beams are not focused at the focal point position of the paraboloid of revolution, a coma is generated by either of the reflective surfaces 133, 135, similarly as in U.S. Pat. No. 3,787,109, but the comas are corrected by canceling them with each other by the symmetry in the arrangement of the reflective surfaces 133, 135. Such an arrangement, however, includes no consideration about the pupil image formation on the pupil of the relay optical system composed of the refracting optical system 132 and the reflective surface 133, and on the pupil at the eye point at all. In order to permit the observer to observe the entire display image at the eye point 136, it is necessary that principal rays at each field angle intersect with each other at the eye point 136. However, because the pupil image formation is not good, the pupil position of the relay optical system composed of the refracting optical system 132 and the reflective surface 133 varies depending upon the field angle. Thus, this arrangement has a problem that a decentering distortion as shown in FIG. 4 appears. Also, because the pupil image formation is not good on the pupil of the above relay optical system and on the pupil at the eye point, the numerical aperture NA on the display surface side of the relay optical system differs depending upon the field angle, resulting in a problem that the observer observes a virtual image with luminance variations.
The optical system as disclosed in Japanese Laid-open Patent Application No. 2-297516 also has the same basic structure and thus has the same problems.
Japanese Laid-open Patent Application No. 5-134208 employs the reflective surface 144 in the shape of ellipsoid of revolution, whereby the pupil image formation is established for the eye point 145 and the relay optical system 142. Further, decentering aberrations generated by the ellipsoid-of-revolution reflective surface 144 are attempted to correct using decentered lenses in the relay optical system 142. However, because the optical system is so arranged that the pupil 143 of relay optical system 142 is inclined a lot relative to the principal ray of the axial beam, correction of aberrations due to the decentering of the relay optical system 142 becomes more difficult as the field angle becomes wider. Especially, correction of distortion is difficult and thus it is not suitable for apparatus required to have a wide field angle. The distortion in such an arrangement is asymmetric as shown in FIG. 5 because of the decentering of the optical system. Thus, supposing two same optical systems are symmetrically arranged for binocular vision, a problem is that it becomes difficult for the observer to achieve fusion of left and right virtual images. Another problem is low degrees of freedom on arrangement of the relay optical system etc., because the aperture position 143 of the relay optical system 142 is restricted by the ellipsoidal shape of the reflective surface 144.